High-resolution structure of an adenosine receptor

Figure: High-resolution structure of the A2A Adenosine Receptor, where sodium is shown in violet.

Ray Stevens group (The Scripps Research Institute) and collaborators

Ray Stevens' group determined a high-resolution structure of the A_2A adenosine receptor, which revealed the structural basis for allosteric modulation of GPCRs by sodium ions. During the past 40 years, allosteric modulation by Na⁺ has been observed for many GPCRs and was linked to motifs in helix II, including the highly conserved Asp^2.50. Mutation of this residue has been the subject of many studies on a multitude of receptors, which have shown that the Na⁺ effect was largely abrogated when the residue was mutated to alanine or asparagine. Despite this indirect evidence, the nature of Na⁺ interactions with GPCRs remained hypothetical, and the sodium ion remained undetected in crystal structures of GPCRs solved at medium resolution. The researchers reengineered the human A_2A adenosine receptor (A_2AAR) by replacing its third intracellular loop with apocytochrome b562RIL and solved the structure to 1.8-Å resolution. This unprecedented high-resolution GPCR structure allowed them to identify 57 ordered water molecules inside the receptor comprising three major clusters. The central cluster harbors a putative sodium ion bound to the highly conserved aspartate residue Asp^2.50. Additionally, two cholesterols stabilize the conformation of helix VI, and one of 23 ordered lipids intercalates inside the ligand-binding pocket. These high-resolution details shed light on the potential role of structured water molecules, sodium ions, and lipids/cholesterol in GPCR stabilization and function. Furthermore, this antagonist-bound structure shows that the Na⁺ ion is part of a water channel that spans the whole receptor; comparison with the active state structure shows dramatic rearrangement in the Na⁺ binding pocket.

Citation:
Liu, W, Chun, E, Thompson, AA, Chubukov, P, Xu, F, Katritch, V, Han, GW, Roth, CB, Heitman, LH, AP, IJ, Cherezov, V, Stevens, RC. Structural basis for allosteric regulation of GPCRs by sodium ions, Science 337, 232-236 (2012).